1. Field of the Invention
The invention relates to a front suspension system for cycles, and finds particular application to motorcycles, although it can be used for other types of cycles, scooters, motorized bicycles, etc.
2. Description of Prior Art
Several types of front suspensions for motorcycles are known. The type of suspension which is by far most popular is the telescopic fork system which comprises a fork connected on top to a steering column and comprises, at the lower portion, two tubes on which two shock absorber casings slide. Other systems have been used, but have been, as of this time, practically abandoned, such as parallelogram forks or balancing forks.
All of the above systems suffer from the major disadvantage arising from their imperfect behavior during braking. This disadvantage is analyzed below with reference to FIGS. 1, 2, and 3 of the drawings. As may be seen from FIG. 1, a chassis 1 of a conventional motorcycle is schematically shown, which comprises a steering column 2 in front and a telescopic fork 3, at the bottom of which is journalled the axle of the front wheel O. In the rest position, or when the motorcycle travels along a straight line at a constant speed, the resultant of the gravitational forces P.sub.r is a vertical force directed downwardly which applies itself at the center of gravity G of the vehicle. This force is equilibrated by two vertical reaction forces P and P' directed upwardly, which apply themselves on the axles of the front and rear wheels. The motorcycle includes a line called the average attitude L, which is substantially horizontal. It is known that the center angle is defined as the angle .alpha. which the axis, around which the rotations making it possible to steer the vehicle forms with the vertical. In the case illustrated, this angle is defined by the inclination of the steering column 2, and is related to the inclination towards the rear of the telescopic fork 3.
FIG. 2 schematically illustrates the front of the motorcycle with its medium attitude line L at rest. When the driver exercises a relatively powerful braking action, two effects are noted: In the first place, a mass transfer occurs towards the front and overloads the front wheel, which results in the appearance of a supplementary reaction force P.sub.i on the front axle O, which assures a new equilibrium. The front shock absorbers are compressed, and the motorcycle inclines itself strongly frontwardly, with a new attitude line L'. With respect to a reference point moving with the motorcycle, the front wheel appears to have undergone a displacement towards the rear, and the front point C of the chassis is displaced downwardly, as shown in FIG. 2. The caster angle of .alpha..sub.2 is strongly diminished.
The braking furthermore results in the creation of a horizontal braking force F, which applies itself on the axle of the front wheel O and is directed towards the rear. This force F tends to increase the compression of the front suspension and the rocking of the motorcycle. To allow for a better understanding of this phenomenon, it may be imagined that the front point C of the chassis has a fixed position. FIG. 3 has been drawn using this reference system showing the trajectory .theta. of the front axle O in the course of suspension movements. This trajectory .theta. is directed towards the rear and upwardly in the case of telescopic suspensions (which takes into account the fact that an increased braking causes a displacement of the axis O towards the rear and a raising of this axis with respect to the chassis, this, in the reference system referred to above where C is taken to be fixed. Of course, since in reality it is the ground which constitutes the horizontal fixed reference, this raising of the point O occurs, in fact, by a descent of the chassis towards the ground).
As may be seen from FIG. 3, the projection f of the braking force F on the tangent to the trajectory .theta. is oriented in the same direction as the projection p of the weight overload P.sub.i and adds itself thus to this latter to cause the displacement towards the rear and upwardly of the point O on the curve .theta.. This illustration of the phenomenon explains the very strong variation of attitude which occurs in practice on known motorcycles during a powerful braking, and which is accompanied by a strong diminution of the caster angle .alpha.. The equilibrium occurs on the trajectory .theta. as a function of the braking power when the supplemental return force developed by the shock absorbers in the compressed state has a projection on the tangent to the curve .theta. sufficient to equilibrate the projections p and f.
The substantial variations of the attitude of the motorcycle and of its caster angle during braking represent a very serious defect in practice. They occur, in effect, when the motorcyclist has initiated a braking action, and thus at the most critical moments. In the first place, they involve rocking movements of the driver with possible oscillations.
It should be noted that these very annoying movements appear with notable amplitude in the case where the braking occurs upon entering a curve. In effect, when turning, the motorcycle is subjected to a supplementary force constituted by the centrifugal force which, as with the gravitational forces, applies itself in a uniform fashion to the entire motorcycle body without any transfer of mass. This force tends to lower the attitude line of the motorcycle while maintaining it parallel to itself. The attitude of the motorcycle during turning is thus very different from the attitude in the course of braking. As a result, at the end of the braking phase, when the motorcycle begins to turn, it undergoes a sudden variation of attitude to pass from the braking attitude to the curving attitude which is particularly annoying at the instant when the cyclist is negotiating a curve.
Furthermore, as has been seen, the caster angle is diminished considerably in the course of braking. Yet, it is known that, for a motorcycle, the lower its caster angle the more unstable it is, such that, at the critical braking moment, the change of attitude of the motorcycle is accompanied by greater instability.
Furthermore, changes of attitude are accompanied by variations in load on the tires with an accompanying risk of loss of traction.
Attempts have been made to reduce the amplitude of these changes of attitude by systems known as "anti-dive" systems. In general, these systems consist of adding a damping control circuit to the braking circuit which, in the course of braking, blocks the displacement of the axle of the wheel with respect to the chassis above a predetermined level. However, these relatively complex systems suffer from the serious inconvenience of totally suppressing the benefit of the suspension when they are blocked, such that the motorcycle no longer has the ability to elastically absorb the shocks imposed by the irregularities of the ground, and to soften these effects as felt by the driver.